linux-kernel - Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD

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Message-ID: <CAMuHMdU3hWDOWXxuOJcBA7tphBT7X-0H+g0-oq0tZdKw+O5W3A@mail.gmail.com>
Date: Fri, 7 Nov 2025 09:35:03 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Ping-Ke Shih <pkshih@...ltek.com>
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Subject: Re: [PATCH v6 12/26] bitfield: Add less-checking __FIELD_{GET,PREP}()

Hi Ping-Ke,

On Fri, 7 Nov 2025 at 02:16, Ping-Ke Shih <pkshih@...ltek.com> wrote:
> Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > The extra checking in field_prep() in case the compiler can
> > determine that the mask is a constant already found a possible bug
> > in drivers/net/wireless/realtek/rtw89/core.c:rtw89_roc_end():
> >
> >     rtw89_write32_mask(rtwdev, reg, B_AX_RX_FLTR_CFG_MASK, rtwdev->hal.rx_fltr);
> >
> > drivers/net/wireless/realtek/rtw89/reg.h:
> >
> >     #define B_AX_RX_MPDU_MAX_LEN_MASK GENMASK(21, 16)
> >     #define B_AX_RX_FLTR_CFG_MASK ((u32)~B_AX_RX_MPDU_MAX_LEN_MASK)
> >
> > so it looks like B_AX_RX_FLTR_CFG_MASK is not the proper mask for
> > this operation...
>
> The purpose of the statements is to update values excluding bits of
> B_AX_RX_MPDU_MAX_LEN_MASK. The use of B_AX_RX_FLTR_CFG_MASK is tricky, but
> the operation is correct because bit 0 is set, so __ffs(mask) returns 0 in
> rtw89_write32_mask(). Then, operation looks like
>
>    orig = read(reg);
>    new = (orig & ~mask) | (data & mask);
>    write(new);

Thanks for your quick confirmation!
So the intention really is to clear bits 22-31, and write the rx_fltr
value to bits 0-15?

if the clearing is not needed, it would be better to use
#define B_AX_RX_FLTR_CFG_MASK GENMASK(15, 0)

If the clearing is needed, I still think it would be better to
change B_AX_RX_FLTR_CFG_MASK, and split the clearing off in a separate
operation, to make it more explicit and obvious for the casual reader.

> Since we don't use FIELD_{GET,PREP} macros with B_AX_RX_FLTR_CFG_MASK, how
> can you find the problem? Please guide us. Thanks.

I still have "[PATCH/RFC 17/17] rtw89: Use bitfield helpers"
https://lore.kernel.org/all/f7b81122f7596fa004188bfae68f25a68c2d2392.1637592133.git.geert+renesas@glider.be/
in my local tree, which started flagging the use of a discontiguous
mask with the improved checking in field_prep().

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds